1. Field of the Invention
The present invention relates to an electromagnetically-actuated valve used as a fuel intake or exhaust emission valve of an internal combustion engines, or more in particular to a method of detecting a fault in such an electromagnetically-actuated valve.
2. Description of the Related Art
The conventional intake/exhaust valve of an internal combustion engine is generally operated by a camshaft driven based on the rotation of a crankshaft. In order to achieve an optimum valve operation time in accordance with the operating conditions and thereby improve the performance of the internal combustion engine, various variable mechanisms for the valve gear system including a two-stage switching system (on/off control system) and a continuously variable system have been developed for practical applications. Some of these variable mechanisms displace the rotational phase of the camshaft and others comprise a plurality of cam profiles of the camshaft.
With the above-mentioned intake/exhaust valves driven by the camshaft, however, all of the valve lift, the valve-open time length and the valve operation timing cannot be set independently and arbitrarily. In an effort to meet the demand for a higher performance of internal combustion engines in recent years, a research effort has vigorously been made to develop an electromagnetically-actuated valve gear system in which these parameters can be set to an ideal value in accordance with the operating conditions.
For example, JP-A-61-250309 (corresponding to U.S. Pat. No. 4,823,825) describes an electromagnetically-actuated valve having a structure in which a valve body supported at the neutral position by an energizing force of a pair of springs is moved from the neutral position to full open or full close position by exerting the electromagnetic force on a plunger coupled to the valve body. The same patent especially discloses a method of detecting a fault of this type of valve. The fault detection method comprises the steps of monitoring the change in the current flowing in a coil when power is supplied to the electromagnetically-actuated valve, deciding that the valve operates normally when the current decreases during a predetermined period, and a deciding that the valve operation is faulty when the current does not decrease during such a period.
The reason of the current decrease is described below. The magnetic flux, i.e., the circuit inductance, is inversely proportional to the square of the distance between the plunger and the core of the electromagnet, and theoretically suddenly increases immediately before the plunger touches the core. With an increase in the magnetic flux, the counter electromotive force e increases according to the formula EQU e=-d.psi./dt=-N(d.PHI./dt)=-L(di/dt) EQU .psi.=N.PHI.
where .psi. is the number of flux interlinkages, .PHI. is the magnetic flux, N is the number of turns, L is the inductance, i is the current and t is the time. The source voltage thus is substantially offset by the counter electromotive force and is not substantially used for supplying current, resulting in a decreased current.
This phenomenon, however, does not always occur. Depending on the hardware configuration including the material and geometry, for example, the magnetic fluxes may saturate at an early time. In such a case, even if the plunger is normally attracted to the core, the current may not decrease. Also, in the final positional control operation for decreasing the current immediately before the plunger almost touches the core, the current may not decrease.